1. Field of the Invention
The present invention relates generally to the construction of III-nitride devices using epitaxial growth, and relates more particularly to the construction of III-nitride devices where the epitaxial growth is variable in direction to obtain variable doping profiles without implantation.
2. Description of Related Art
In the construction of semiconductor devices, doping of various portions of semiconductor material is achieved according to a number of different techniques. One of the more popular techniques is the implantation of ions using a beam, and driving the implanted impurities to diffuse into the semiconductor material in which the implant is made. The implantation of ions according to these typical techniques often involve high energy collisions between the implanted ions and the lattice structure of the semiconductor material to be implanted. Accordingly, structural damage to the lattice of the semiconductor material is a typical and well known byproduct of the doping process.
As integrated circuit become more sophisticated, complicated doping profiles and geometries are used to achieve better performance and critical parameter values. However, as more complicated implantation processes are conducted, a significant amount of structural damage to the implanted semiconductor material is observed. The structural damage to the lattice of the implanted semiconductor material tends to degrade the performance of the devices in certain critical areas, such as breakdown voltage.
Power semiconductor devices are often constructed with MOSgated switches to take advantage of the low on resistance to reduce power losses. MOSgated switches are typically constructed using ion implantation, as discussed above, and are usually rated for a particular voltage. Accordingly, due to the deterioration of the semiconductor material caused by ion implantation, MOSgated switches typically need to be constructed with additional voltage blocking capacity to compensate for the reduced blocking ability caused by the implantation process. In addition, MOSgated switches are typically specified to have a given current carrying capacity for a given voltage rating. The current carrying capacity is limited by the carrier density in the voltage standoff region. Some geometries have been presented for construction of MOSgated devices that improve the current capacity versus voltage standoff relationship, where charge compensation is provided in the MOSgated switch base. These types of devices are often referred to as superjunction devices. However, the devices continue to be limited by the use of ion implantation for doping and the attendant damage to the lattice of the semiconductor structure, leading to lower breakdown voltages.
III-nitride semiconductors are presently known that exhibit a large dielectric breakdown field of greater than 2.2 mv/cm. III-nitride heterojunction structures are also capable of carrying extremely high currents, which makes devices fabricated in the III-nitride material system excellent for power applications. Devices fabricated in the III-nitride material system can exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFETs), high electron mobility transistors (HEMTs) or modulation doped field effect transistors (MODFETs). These types of devices typically operate through the use of piezoelectric polarization fields to generate a two dimensional electron gas (2DEG) that allows transport of very high current densities with very low resistive losses. The 2DEG is formed at an interface of two III-nitride material layers having different concentrations of III-nitride materials. Due to the nature of the interface, fundamentally formed III-nitride semiconductor devices tend to be nominally on, or depletion mode devices.
Materials in the III-nitride material system may include gallium, aluminum and indium, as well as their nitrides, GaN, AlN and InN. Gallium nitride and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN) are also included in this material system. These materials represent semiconductor compounds that have a relatively wide direct bandgap that permits highly energetic electronic transitions to occur. Gallium nitride materials have been formed on a number of different substrates including silicon carbide (SiC), sapphire and silicon. Silicon substrates are readily available and relatively inexpensive, and silicon processing technology is well developed. Epitaxial growth of III-nitride materials to form semiconductor structures has also been well developed, and results in decreased complexity in manufacturing, as well as providing superior thermal performance.